Packaging containers of paper or paperboard which are intended to be filled with a food product which, with a view to extending its shelf-life, is to be subjected to a thermal treatment in the closed packaging container are generally filled only partly with the relevant product in order to permit a pressure-equalising expansion of the product during the heat treatment. Such an expansion possibility is particularly important in those cases when the food product consists wholly of or at least partly includes a liquid fraction which may readily be vaporized and thereby rapidly raise the vapour or steam pressure inside the packaging container to a level where the packaging container may be ruptured in its sealing joints, or even explode. A further reason for only partly filling the packaging container is that the free space above the level of the product can be efficiently utilised for accommodating such volume variations in the product flow as readily occur in the event of defects in a filling machine.
An unfilled space (so-called headspace) above the level of the product in a packaging container filled with a food product may, however, be associated with serious risks for the packed product, since ambient air in contact with the product flow before and during the filling operation proper may readily be entrained by and accompany the product flow into the packaging container and there accumulate and be entrapped in this space. Contact between such an entrapped volume of air and a packed food product inevitably leads to a harmful oxidative action on the product with the result that the product, by oxidation with the oxygen gas of the air, can be changed or deteriorate in both flavour and aroma as well as in appearance, and thereby become unattractive and even unfit for consumption. Such an oxidative process of destruction takes place over time and is different for different types of food products. For particularly oxygen gas-sensitive products, such as potatoes, asparagus, tubers, maize, fruits, meat and pet food, such a degradation process takes place very rapidly, while other less oxygen gas-sensitive products can, on the other hand, survive for a longer time without any appreciable negative effect.
Regardless of whether the food product which is to be packed and, by means of a subsequent heat treatment, be given extended shelf-life in the packaging container is of the one or other type, it is desirable to carry out the filling operation under such conditions that the entrapped volume of air in the packaging container will be as slight as possible and most preferably totally eliminated.
According to one prior art method, the product filling is carried out under the supply of a non-oxidising gas, such as, for example, nitrogen which, via one or more nozzles, is injected into the packaging container before and/or during the entire product filling operation in order to drive out air from the packaging container and fill the headspace above the level of the product with this gas. One example of such a prior art method is described in U.S. Pat. No. 3,477,192. A filling, during simultaneous supply of non-oxidising gas, requires immediate closure of the packaging container after filling in order to prevent the renewed entry of air into the packaging container. In practice however, product filling most generally takes place at a different site than subsequent closure of the packaging container filled with product, which thus implies that the filled, and as yet open packaging container, must first be conveyed a certain distance with the filled product in contact with the ambient air before the packaging container is closed. Even if the time for such a transport between filling and closure can be made very short, there is nevertheless always a risk that at least a part of the injected non-oxidising gas will have time to be forced out and replaced by air which has time to flow into the packaging container during this brief transport. Such inflowing air will hence be enclosed in and accompany the packaging container in direct contact with the product during the further storage of the product in the unopened packaging container.
According to a prior art method which is described in EP 0008886 A1, the risk of accompanying air in a closed packaging container may effectively be prevented or at least considerably reduced if the packaging container, before and/or in association with the product filling operation, is filled with a foam containing a non-oxidising gas, e.g. nitrogen. According to EP 0008886 A1, the open packaging container may thus first be filled the gas-containing foam in order to force out air present within the packaging container, and thereafter be filled with product to the desired level. After this filling, the headspace above the level of the product is more or less completely filled with foam which remains inside, and thus accompanies the packaging container after closure. The thus foam- and product-filled packaging container may thereafter be subjected to a heat treatment with a view to extending its shelf-life (pasteurization), i.e. a heat treatment during which the product is briefly heated to and held at a temperature of at most approx. 100° C., normally 72-78° C. According to EP 0008886 A1, it is also possible first to fill the open packaging container with the pertinent product to the desired level and thereafter, or in immediate association with this product filling, fill the remaining headspace above the product level with the gas-containing foam for forcing out any residual air. The thus foam- and product-filled packaging container is thereafter closed and sealed and can, as above, be subjected to a pasteurization for extending its shelf-life.
Examples of products which may be packed and heat treated in the method according to EP 0008886 A1 are liquids such as fermented beverages such as beer and non-carbonated drinks such as still drinks. However, EP 0008886 A1 is silent as to whether foods of a solid or semi-solid nature, such as for example potatoes, asparagus, tubers, maize, fruit, vegetables, meat and pet food can be used in the prior art method. Nor is it apparent from EP 0008886 A1 whether such solid or semi-solid foods can be packed and heat treated in combination with liquids, such as sauces, cooking juice, stock etc.
One drawback inherent in the prior art method according to EP 0008886 A1 is, however, that the foam entrapped in the product filling has a tendency to remain in and accompany the packaging container more or less intact, even after the subsequent heat treatment (the pasteurization) and the further storage of the product until it is time to open the packaging container. The remaining foam which is exposed on opening of the packaging container may thus give the packed product an excessively frothy appearance which can be associated with a deterioration in product quality caused by defective storage of the product during its storage time in the packaging container. The problem inherent in excessive froth or foam in the opened packaging container will naturally be more or less manifest in those cases where it is not at all expected that the packed product should froth, or even be remotely associated with froth, such as, for example, non-carbonated still drinks.
Hence, there is still a need to be able to pack and heat treat a food product with a view to extending its shelf-life in packaging containers of paper or paperboard without the risk of entrapping air or oxygen in the packaging container during the product filling and without the risk of consequential negative effects on the appearance of the filled product which is exposed on opening of the packaging container. In particular, there is still a need in the art to be able to pack a food of solid or semi-solid nature together with liquid foods, such as, for example, sauces, cooking juice, stock etc. without the risk of such harmful enclosure of air or oxygen and without the risk of such negative effects on the product which has been packed and heat treated for purposes of extending its shelf-life.